sean collier is caught on the campus at 10:20. when do the gun shots and explosions in watertown start? >> they start approximately 12:25 a.m. >> what is the first thing you hear about? how do you know there are gun shots? >> i like to tell the officer's story. they were on the street and we knew there was the execution of the mit police officer. we knew there was a car jacking and as a result of that vehicle we had gps updates from cambridge telling us where the vehicle was. it was heading ward watertown and that it was actually in watertown. that's when we began to go into watertown to find the vehicle in the east. >> were your officers alone at that time? were you assisted by the fbi and atf state police were joined by the hunt? >> all of the ages agencies wer given the same update we were in boston and cambridge it was our officers who encountered the vehicle first. other officers responded behind us. >> when they encounter that vehicle, chief, tell us exactly what happened. >> first officer, officer joe reynolds saw the black